The present invention relates to an apparatus for reproducing digital signal information in a compact disk player, digital audio tape deck, etc.
The tremendous supply of digital audio apparatus such as a compact disk player in markets in recent years has permitted easy amusement by general users.
The conventional digital audio apparatus will be explained in an example of a compact disk player.
FIG. 4 is a block diagram of the compact disk player. Using this figure, a brief explanation will be given for general flow of digital signals. Rotating a spindle motor 1 rotates a disk 2; then an optical pickup 3, arranged near the disk 2, reads digital signals recorded on the disk 2. One of them, referred to as an RF signal, is sent to an analog waveform signal shaping block 4, while the other of them, referred to as a servo signal, is sent to a servo signal block 5. The servo signal contributes to good continuous reading of pits of data engraved on the disk 2. The RF signal, passed through the analog signal shaping block 4, changes into a pulse train waveform in accordance with the information recorded on disk 2, which is converted into an impulse waveform by a PLL circuit block 6. The impulse waveform is a pulse train waveform corresponding to a clock frequency owing to correlation with a synchronous detection circuit block 7; this pulse train waveform is sent to a digital signal processing block 8. The clock frequency generally determined is actually 4.3218 MHz; it is now assumed to be fi-Hz for convenience of explanation.
The data subjected to digital processing by the digital signal processing circuit block 8 is once stored in a RAM block 9, and thereafter it is returned to the digital signal processing circuit block 8 in synchronism with a clock pulse provided by a quartz oscillator therein. The resultant data is supplied to a D/A converter circuit block 10 for its D-A conversion. The analog signal thus formed is outputted as a left (L) channel signal and a right (R) channel signal.
The RAM 9 generally has a large capacity of 16K bits. Data is inputted or outputted from the RAM 9 at a very high speed; the signal (data) is once stored in the RAM 9 and thereafter read out instantaneously therefrom. This contributes to reducing the delay time, i.e., the time from when the optical pickup starts to read the signal on the disk 2 to when the read signal is outputted as an analog signal.
FIGS. 5A, 5B and 5C show time changes in numerical values of main concepts in the prior art. In the abscissa, the time t0 represents a disk play starting time, the data read start time, the start time of disk reproduction, and the data store start time. Time t3 represents a disk play ending time and the data output completion time.
FIG. 5A illustrates time changes in RAM capacity. The ordinate represents the amount of data actually stored in the RAM 9; N1 corresponds to bits of ten thousand and several thousands. Strictly speaking, a time lag occurs from the time t0 to the time when N1 is reached; this time lag can be disregarded since it is 0.01 sec or less. The time delay from start of read of data to production of analog signals (actual sound production), which actually occurs, can be also disregarded since it is very short. N1, illustrated constant from the time t0 to t3, actually changes continuously when viewed in a small scale; it is illustrated as shown in FIG. 5A since it is averagely constant.
FIG. 5B shows time changes in the clock frequency in the PLL circuit block 6. The clock frequency remains constant f1 from the time t0 to t3. The clock frequency in reading out the data once stored in the RAM 9 as mentioned above is set to be equal to the above clock frequency in the PLL circuit block 6. Therefore, a predetermined amount of data is continuously stored in or read from the RAM 9.
FIG. 5C shows time changes in the linear speed of the disk at the position of the optical pick-up. This linear speed is equal to the speed of reading the data on the disk. In the case of a compact disk player, this linear speed S1 is set to be 1.2 to 1.4 m/s. Namely, the linear speed S1 is constant from the time t0 to t3. This was the feature of the compact disk player.
The above arrangement in which the disk playback time means a playing time as it is has the following drawback. It takes a time to start the playback of another disk after having enjoyed a piece of music, etc. Specifically, extraction of one disk from the spindle motor and setting of another disk requires a loading mechanism, so that actual exchange of disks takes several seconds (usually at least ten).